Large screen CRT displays may require a high amount of inner raster distortion correction for adequate raster linearity. The commonly used east-west pincushion distortion correction circuit using a diode modulator can correct some inner pincushion distortion. It has been found, however, that the correction of larger inner raster distortion errors is difficult. Furthermore, because prior art correction of inner raster distortion is difficult or impossible to be adjusted, the results are often unsatisfactory.
Horizontal deflection circuits commonly employ resonant trace and retrace principles. During retrace, the deflection circuit forms a high frequency parallel resonant circuit: energy is supplied to the deflection circuit from the flyback transformer primary winding. During the trace interval, a deflection switch provides a low frequency series resonant circuit formed by the deflection yoke winding and the S-shaping capacitor. As a result, energy circulates from the yoke to the S-capacitor and back to the yoke in one trace period. This produces a sinewave shaped deflection current with a conduction angle of approx. 130.degree., starting at about 115.degree. and ending at about 245.degree.. The retrace provides a fast return of the deflection current from the nominal 245.degree. position at the end of trace to the nominal 130.degree. position at the start of trace.
The value of the S-capacitor determines the conduction angle or the S-shaping of the deflection current. Thus, the S-capacitor is selected for best horizontal linearity. The voltage across the S-capacitor is cosine shaped. The amplitude is highest at the center of trace, when the deflection current is zero. The correction of inner raster distortion requires modulation of the S-shaping as a function of the distance along the vertical axis of the raster display, i.e. from top to the center and from the center to the bottom of the raster. This is achieved by modulation of the voltage across the S-capacitor. The modulated voltage appears also across the horizontal yoke and causes a modulation of the deflection current.